Robust exchange interactions in silicon from valley polarization and filtering

Heisenberg exchange is a key process for entangling single spin qubits and defining logical spin qubits in silicon. However, the role of valley degrees of freedom in exchange is not yet experimentally understood. Here we spatially map the exchange interaction between a single donor atom and a single-electron quantum dot that can be positioned with sub-nm precision using a scanning tunneling microscope. Exchange is found to vary smoothly in space, due to the disorder-free interface for the dot. Lattice-incommensurate exchange oscillations are not detected to < 1 % of the nominal exchange energy, because the x and y valleys of the donor are filtered out of donor/dot tunneling, due to the dots’ z-valley polarization. The demonstrated valley filtering is of interest for valleytronics, and the absence of valley-induced disorder and rapid exchange oscillations is of interest to design robust Si-based quantum computers.